Brain arteriovenous (AV) malformations (BAVMs) can cause life-threatening strokes and have limited treatment options. The goal for this project is to elucidate the cellular and molecular mechanisms underlying BAVM pathogenesis to identify novel candidates as therapeutic targets to ameliorate this disease. AVMs are characterized by abnormal AV shunts that displace intervening capillaries. We propose a cross-disciplinary approach, fusing cutting-edge mouse genetics and imaging technologies, to test our hypothesis that Notch mutations can reprogram AV identity and alter AV differential nitric oxide (NO) signaling, and thus endothelial dysfunction, to elicit BAVMs. Notch receptors and ligands are expressed in arteries but not veins. Notch signaling promotes arterial at the expense of venous differentiation by enhancing arterial and suppressing venous molecular markers. We have reported that endothelial expression of a constitutively active Notch4 mutation (Notch4*) elicits BAVMs in mice. Notch4* reprograms veins to gain arterial and lose venous molecular identity, and correcting the causal Notch4* leads to normalization of established BAVMs. We have built a custom two-photon microscope, optimal for structural and hemodynamic imaging of cerebral vasculature in live mice. We can thus obtain 5D data (3D plus blood velocity over time) through a cranial window to reveal the process of BAVM formation in mice. Built on our strong background and preliminary data, we propose: Aim 1 - Determine the effect of endothelial Notch4* on venous endothelial dysfunction and BAVM formation. We will test our hypothesis that Notch4* upregulates NO levels in the veins, alters venous endothelial response to blood flow, and thus permits AVM formation. We will examine the effect of Notch4* on venous NO signaling, endothelial response to blood flow, and flow mediated BAVM formation; Aim 2 - Determine the effect of endothelial Notch deficiency on arterial endothelial dysfunction and BAVM formation. We will test our hypothesis that loss of endothelial Notch gene function reduces arterial NO signaling, leading to arterial dysfunction and thus AVMs. We will analyze mice with endothelial deletion of Rbpj for BAVM pathology, arterial NO signaling, and endothelial response to blood flow stimuli; Aim 3 - Compare Notch and Alk1 mouse mutants in DA and CV development and BAVM formation. We will test our hypothesis that Notch interacts with Hereditary Hemorrhagic Telangectasia (HHT) 2 (or Alk1) in AV differentiation and AVM formation. We will compare the Notch and HHT mutant phenotypes using two-photon imaging and 3D rendering and perform genetic rescue. The findings from this study will conceptually advance our understanding of the cellular and molecular mechanisms of AVM pathogenesis, reveal novel functions for Notch in regulating the unique physiology of arteries and veins, and uncover interactions between the Notch and HHT pathways. The success of this work will inspire new areas of investigation in the fields of AVMs, Notch signaling, and vascular pathophysiology.